1. Field of the Invention
This invention relates to a flaw detection of a steel material and, more particularly to an AC magnetic flux leakage type flaw detecting apparatus adapted to detect a flaw on a flat surface of a thick steel plate or an H- shaped steel.
2. Description of the Prior Art
As a non-destructive detecting method widely used in an inspection of a quality of a primary product of a steel plate, there are known certain methods such as an ultrasonic flaw detecting method, a magnetic flaw detecting method, an eddy current flaw detecting method, a magnetic flux leakage flaw detecting method, etc. According to the ultrasonic flaw detecting method and the magnetic flux leakage detecting method, there are problems that, since water, fluorescent magnetic powder/liquid are employed and an eye observation inspection are used, its low processing capacity and environmental pollution occur. The eddy current flaw detecting method has an excellent processing capacity due to fast detecting speed, but since it requires a magnetic saturation unit and a de magnetizing unit, the shape of the material to be detected is limited to a columnar or cylindrical shape, but it is scarcely applied to a flat surface of the plate except hot eddy current flaw detection exhibiting no ferromagnetism. Further, the leakage magnetic flux flaw detecting method is used to mainly detect a flaw of a round rod steel or steel pipe. This method detects a flaw by conveying a material to be detected of columnar or cylindrical shape while spirally rotating it on a turning roll, and disposing a sensor directly under the material to be detected. According to this method, the material to be detected is spirally rotated to detect the flaw on the entire surface. The whole surface of the material to be detected can also be flaw detected by linearly moving a round rod steel or a steel pipe by a conveyor and rotating an exciting yoke and a sensor along the outer periphery of the material to be detected. However, when the flaw of a square billet is intended to be detected, the material must be conveyed linearly. Thus, this method has such a disadvantage that the detecting region of the sensor must require the width at each surface of the billet and its detecting element must have multi-channels. Further, this method also has a defect that a flaw undetected range is generated on the ed of the outer periphery of the material to be detected.
As described above, the conventional steel material non-destructive detecting methods all have problems in the processing capacity or the environmental pollution, or in a relatively large scale due to the necessity of a magnetic saturation unit or a degaussing unit. Further, the material to be detected is limited to the columnar or cylindrical shape, and the conventional method is not adapted for detecting a flaw of the flat surface of the thick steel plate or the H-shaped steel and a thick plate welded portion. Thus, there is no conventional method which can defect a flaw of the thick steel plate or H-shaped steel by a simple mechanism without problems of the insufficient processing capacity and environmental pollution and without flaw undetected range. Generally, since side edges and its peripheries of the thick steel plate frequently suffer from defects due to rolling process, the necessity of detecting the defect is probably high, but according to the conventional magnetic flux leakage flaw detecting method, there are problems because undetected range exists at ends of the material to be detected and a linear flaw generated along the rolling or conveying direction frequently at the ends cannot be detected.
An object of this invention is to provide an AC magnetic flux leakage type flaw detecting apparatus for use in flat surface flaw detecting which can eliminate the above-described problems of the conventional art.